The current practice for shutting a well is to first close the annular BOP so that it grips, and forms a seal with, the drill string. Once a seal is established around the drill string, the drill string can be sheared by the BOP shear rams, and a seal formed across the well.
This procedure is complicated by the presence of tool joints, that is a joint between, for example, multiple sections of drill string. A tool joint presents a mass of material which the BOP shear rams would have difficulty cutting, presenting a potential problem in the way of sealing the well.
The current practice for shutting a well is to first close the annular BOP. The drill string is then pulled until an increase in hook load is detected. This indicates that a tool joint has hit the annular BOP seal and therefore the position of the tool joint is established. After this the tool joint can be positioned so that the BOP shear rams can be deployed.
This process is time consuming and requires judgement from the driller over the position of the tool joint.
Being able to give the driller real time data on the position of tool joints would be a great advantage both in time saving and reliability.